Most commercial recycle processes for polyethylene terephthalate (PET) containers require that any label be completely removed and separated, in order to minimize contamination and provide a high quality recycle flake. This is because the inks in the label, as well as the label itself (if made from a non-polyester material), have a negative impact on PET and can cause haze and discoloration.
A common method for achieving some or all of this separation is to subject the materials to a “sink-float” separation in a water-based wash bath. With traditional, non-shrink labels made from polypropylene, this approach works well since the polypropylene has a density less than 1 g/cm3 and will float to the top of the bath where it can be removed. In contrast, the PET component has a density of 1.33 to 1.35 g/cm3 and will sink to the bottom where it can be easily collected.
The growing popularity of heat-shrink labels (or shrink labels) has complicated matters, however. Shrink labels typically have a density greater than 1.0 g/cm3 and tend to sink with the PET flake. This is true whether the labels are made from copolyester, polystyrene, poly(lactic acid) (PLA), or poly(vinyl chloride) (PVC). Because these labels and their printing inks stay mixed with the PET at the bottom of the tank, they contaminate the PET and may cause haze/color problems. While some shrink materials, like copolyester, are compatible with PET and can be tolerated at low levels as long as ink contamination is minimized, some label stock (e.g., polystyrene and PVC) can cause additional problems when the PET flake is later reprocessed. For example, at PET processing temperatures, PVC contamination tends to severely corrode extruder barrels due to the release of hydrochloric acid.
There are some solutions to this problem already on the market including micro-voided shrink films, as well as labels that are pre-foamed. These approaches reduce the density of the label stock, making it more easily separable, but also render the label opaque. For some applications, this opacity may be acceptable (and even desirable), but for many packages, the brand owner desires clear or translucent label stock to better show off their product. Furthermore, in the case of pre-foamed labels, it is also very difficult to print on these labels and provide any kind of “eye-catching” graphics given the rough-nature of the surface.
Because of (a) the wide range of shrink labels on the market, (b) the difficulty in rapid identification of label type at the recycle facility, and (c) the serious negative potential of contamination by a material like PVC, most recyclers simply choose to dispose of all shrink labeled bottles rather than try to reclaim the PET flake. Even though some shrink labels tend to not cause major issues for recyclers, the risk that other more-damaging label stock might get through the system is too great. As a result, a large quantity of high-quality PET flake is getting sent to landfills despite the fact that it could be reclaimed.
Thus, there is a need for a film that is suitable for use as packaging labels or wrappings and that can be easily separated from PET flake without adversely affecting the aesthetic characteristics that make such packaging material so popular. There is also a need for a process that can easily separate the film from PET flake. Ideally, the solution would work for all types of film materials, so that recyclers would no longer have to worry about separating acceptable film materials from bad.
The present invention addresses these needs as well as others that will become apparent from the following description and the appended claims. In one embodiment, the invention solves a recycler's problems by using inks/coatings on the packaging label or wrapping material that foams during the recycle flake washing step.